Vacuum processors for processing a workpiece (i.e., etching materials from or depositing materials onto the workpiece) typically include first and second ports respectively connected to a vacuum pump and one or more gas sources. The gas is excited to a plasma in the chamber by an electric source including a reactance responsive to a first AC source, typically an RF or microwave source. A first matching network is usually connected between the first AC source and the reactance for exciting the plasma. If the source is an RF source, the reactance is either a coil for supplying magnetic and electric fields to the chamber interior via a dielectric window or a parallel plate capacitive arrangement for supplying an electric field to the chamber interior.
The workpiece, which is typically a semiconductor wafer or a dielectric sheet or a metal plate, is clamped in piece on a workpiece holder, i.e., chuck, that frequently includes an electrode covered by a dielectric. DC voltage is typically applied to the electrode to provide an electrostatic clamping force to hold the workpiece in situ on the holder. The workpiece is usually cooled by applying a coolant agent, such as helium, to a recess in the chuck and by applying a liquid to conduits in the chuck. To accelerate ions in the plasma to the workpiece, a second AC source is connected to the electrode by way of a matching network. Each matching network includes a pair of variable reactances having values that are varied by motors, typically step motors.
Sensors for electric parameters associated with the plasma, as coupled to the excitation reactance and as coupled to the chuck electrode, derive signals which assist in controlling the values of the variable reactances. Pressure and flow rate transducers respectively in the chamber and in a line supplying gas to the second port derive signals which assist in controlling the vacuum pressure in the chamber and the flow rate of gas flowing into the chamber through the second port.
A controller, including a microprocessor and a memory system including a hard drive, random access memory (RAM) and a read only memory (ROM), responds to the signals derived by the transducers and signals from an operator input console to produce signals for controlling the variable reactances, output power of the two AC sources, the vacuum pressure in the chamber and the flow rate of gases supplied to the chamber through the second port. The memory system stores several recipes, each in the form of signals representing various parameters controlling the deposition and etching of the workpieces for differing situations. The parameters of each recipe are, inter alia, gas species to be supplied to the chamber, flow rates of the species, vacuum pressure in the chamber and output powers of the two AC sources. Each recipe can include other parameters, such as time for carrying out each recipe step. The controller responds to the parameters of the recipe to control valves for the flow of the gases into the chamber, the chamber pressure, as well as the output power of the first and second AC sources. During processing, the controller controls the reactances of the first and second matching networks so that there is an efficient transfer of power between the first and second AC sources and the loads they drive so the impedances seen looking into the output terminals of the first and second sources are substantially equal to the impedances the first and second sources respectively see by looking from their output terminals into cables connected to the first and second matching networks.
Typically, a recipe change has been marked by step, i.e. sudden, changes in at least one of (1) gas flow rate, (2) chamber pressure, (3) power supplied to a plasma excitation coil, (4) the gas species flowing into the chamber, and (5) power supplied to (a) an electrode, such as bottom electrode on which the workpiece is mounted or a top electrode for exciting a gas to a plasma, or (b) RF plasma excitation coil. These step changes result in sharp demarcations between layers etched from the workpiece or deposited on the workpiece. For example, the step changes during etching of a trench in a workpiece, e.g., a silicon substrate, result in sharp corners between a wall and base of the trench. Such step changes also frequently result in sharp corners at a boundary between a trench wall and a layer at the top of the trench. Such sharp corners can make it difficult to fill the trench during subsequent operations and have other known disadvantages, such as causing stress related defects and/or electrical leakage.
One method of addressing the problem which has resulted in somewhat smooth transitions when certain recipe changes are made involves adding dilutants, such as argon or helium, or passivation gases, such as oxygen, on a transient basis, to gases flowing into the processing chamber during a process recipe step occurs. However, there are disadvantages in transiently adding dilutant and/or passivation gases to the processing chamber. Because of the relatively large volume of a typical plasma processing chamber, a significant amount of time, up to ten seconds, is required to purge “old” gas from a line coupling gas from a gas source into the chamber. As a result, there are substantial increases in workpiece processing time, to reduce chamber efficiency and decrease workpiece throughput. In addition, changing the gas species on a transient basis results in a change in plasma impedance. The change in plasma impedance adversely affects the ability of the matching network between the electric source and the coil and/or electrode to provide an efficient transfer of power between the source or sources and the driven loads. In addition, the time for the new gas, i.e., the dilutant or passivation gas, to flow into the chamber is likely to vary as a function of gas line length between the chamber and the gas source. As a result, precise control of the processing step is difficult to achieve and/or recipe processing steps must be customized for the different gas line lengths between the different gas sources and the chamber.
Chen at al, U.S. Pat. No. 5,807,789 discloses a method of operating a plasma processor to form in a semiconductor workpiece a shallow trench with a tapered profile and round corners. Such a shallow trench is formed during successive recipe steps. During a first step the plasma power and chamber pressure are respectively relatively high and low. During the next steps, the plasma power and chamber pressure are respectively decreased and increased. The process continues in this way for at least one additional step.
In a particular etching embodiment Chen et al discloses, the plural gas species applied to the chamber remain the same and at constant flow rates while power supplied to the plasma is reduced in three steps, each of which occurs simultaneously with an increase in chamber pressure. During a first step, which lasts for eight seconds, the power supplied to a plasma excitation reactance and chamber pressure are respectively 800 watts and 50 millitorr. At the beginning of a second eight second step, the supplied power is reduced suddenly from 800 watts to 750 watts while chamber pressure is increased suddenly to 80 millitorr. At the beginning of a third 46 second step, supplied power is suddenly reduced further to 650 watts while chamber pressure is suddenly increased to 100 millitorr.
The aforementioned process suffers from similar problems to the previously mentioned problems associated with adding dilutants because of the substantial time required to change pressure in the relatively large volume vacuum chamber. In addition, the sudden power changes frequently do not enable the corners to be rounded to the desired extent.
It is, accordingly, an object of the invention to provide a new and improved method of and apparatus for operating a vacuum plasma processing chamber.
An additional object of the invention is to provide a new and improved method of and apparatus for operating a vacuum plasma processing chamber in such a manner that sharp corners on processed workpieces are avoided.
Another object of the invention is to provide a new and improved method of and apparatus for controlling a vacuum plasma processor such that changes in a processing recipe are performed in a manner which avoids sharp corners on a processed workpiece.
Still another object of the invention is to provide a new and improved method of and apparatus for processing a workpiece in a vacuum plasma workpiece processor so that changes during a recipe are performed in such a way as to avoid sharp corners in a processed workpiece and wherein processor throughput is relatively high.
Still another object of the invention is to provide a new and improved method of and apparatus for controlling processing of workpieces in a vacuum plasma processor, wherein changes in steps of a recipe are performed in such a way that sharp corners of the workpiece are avoided, without substantial impedance mismatches occurring between one or more sources driving reactive components which supply power to processing gas in the chamber.